Tjwolf92

heya guys hopefully you can shed some light on this for me, my friend has a 97 XJ that has a 99 motor and trans in it. they take different IAC valve's which i believe is what is causing the problem but i'm not positive, any ideas on where I should start to look? it dies almost like its running out of gas every time it gets up to operating temp. let it cool down and it starts right back up no problem. from reading I've seen everything from bad coil packs, crankshaft positioning sensors, TPS, cam positioning sensors, to PCMs, where do i start to begin eliminating some of the plethora of things it could be? thanks in advance! happy jeepin!

Cherockee

If the Crank Sensor isn't new or recent I would start with that. If it doesn't solve the problem just call it preventive maintenance.

It's best to diagnose what is wrong though. What doesn't it have; Air, Fuel, spark, timing? The root cause is one of these things.

tjwalker

Sounds like you are suffering from "thermal" (heat related) failure.

The #1 and #2 causes of that symptom are the crankshaft position sensor and the ignition coil. In that order.

Both can be tested (and you should test for spark as well) and should be tested IMMEDIATELY after it stalls, during the cool down.

Tjwolf92

Okay thanks, how could each be checked? I do have a multimeter. like i said its a friends jeep, he said it is hard to start when cold, once it warms up the idle skyrockets. it revs every time it warms up to operating temp, its not just a slight rev either high enough to make you want to shut it off. so sometimes its shut off due to fear of over revving (since there isn't a tach or exhaust i'm not sure of exact RPM's) and other times it will die out on its own. If this helps any diagnostics, we'll be out checking on it when this damn snow clears up thought it was spring?
Thanks again, and in advance for any help!

Cherockee

Okay, now there sounds like you have several different things going on there.
I would suggest you start with putting the exhaust back on with some O2 sensors. Make sure there are no vacume leaks. If someone has the exhaust off maybe there are some other parts missing, like vacume lines from the intake manifold. Verify all the sensor are in place and getting voltage. Do a tune up if it's due for one (plugs, cap, rotor, oil, coolant). Then lets see where we stand with it's running condition. Once you have it running correctly again start budgeting for sensor replacement (Temp, IAC, Oil, etc.).

Tjwolf92

Today replacing the crankshaft position sensor helped allot, starts better runs better, killer spark, but as soon as it get warm it revs to the moon. but no dying now. if you snap the throttle when it starts to rev up then it revs more and stays there. about to check for vacuum leaks with throttle body cleaner, will report back then. When he got this jeep it ran great, then things started going all to hell. that's the reason im not to concerned with the o2 sensors and exhaust, there is a least one in black and hooked up that i seen today. any help appreciated! thanks in advance

tjwalker

Glad to hear that a new crank sensor solved one problem. Now onto the next.

I'd test the "throttle position sensor". It is the engine management sensor that could be involved with this symptom. Here is the "how-to".

The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

• Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted accordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A fault TPS can report values outside the denied acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

Roblaza

You sir, may have just solved MY issue.

Mine runs good until she's up to operating temp, but the trans has been shifting somewhat funky for the past little while. I hit the gas and she cut out like it was starving for fuel, I let off and she dies. Just started happening today.

I'll look at the tps first as it seems to fit.

Paul Leonard

I read your forum and it really helped my 1996 jeep Cherokee straight 6 did the same thing I use it for plowing snow off road only use and real pain when every 30 minutes engine shut of when it warms up and won’t start. No spark at plugs. Wait 2 maybe 3 hrs it starts right up and 30 minutes later it shuts off again.
After doing tuneup new plugs, wires, coil, pickup coil, swaps relays checked for vacuum leaks, checked all wires , added extra ground from engine to frame and chassis. Still engine shuts off after about 30 minutes.
But then I read your forem. you guys told me to change the CRANK SHAFT POSITION SENSOR And guess what it stays running four ever I plowed snow all night. The sensor must go bad sfter warms up.
Thank you guys. Paul.